Low Energy Electron Diffraction Apparatus Designed for Thin Film Use

Abstract

For the study of thin epitaxial films by LEED it has been necessary to design a vacuum system more versatile than the system now in use. The vacuum and electron optical requirements are basically the same as for previous LEED systems. The additional requirements for thin epitaxial film studies are more precise control of evaporation sources, multiple sources, and thickness monitoring equipment. This paper describes an all-metal system built to meet these requirements. To allow room for a manipulation of evaporation sources, the vacuum system is of the metal bell jar type with directly attached ion-type pumps (100 liters/sec) plus a large area sublimation pump (1500 liters/sec). A mercury diffusion pump system is attached via a bakeable valve for prepumping and noble gas pumping. Three or more evaporation sources can be positioned to individually check their outgassing rate, check their evaporation rate and evaporate on the sample substrate. Rate of evaporation can be checked before evaporation on the substrate by a quartz crystal monitor or ion gauge type monitor. Also the total amount of material evaporated at the sample can be recorded directly or in reference to the change in conductivity of the deposited film. Sample holders can be of two types; one that can be heated to high temperature or cooled close to liquid N2 temperatures or a Dewar type coolable to liquid He temperatures. The two types of holder require a different evaporator arrangement. The high speed of the sublimation pump removes active gases liberated during operation of the evaporator. This eliminates the problem of large changes of background pressure during operation. The diffusion pump allows removal of noble gases after ion sputtering of the sample. Preliminary data obtained from the system indicate that the design is much more useful for thin epitaxial film studies than systems presently in use and it is probable that a similar versatility will be important for other applications.